1. Field of the Invention
The present invention relates to an oxide superconductor in which a superconductive layer in a thin or thick film shape is formed on a substrate, and its manufacturing method.
2. Description of the Background Art
Conventionally known methods of manufacturing an oxide superconductor such as YBaCuO type superconductor include a sputtering method and a vacuum evaporation method. In these conventionally known methods, it is fairly common to have the composition of the obtained thin film to be somewhat different from the composition of the target or the evaporation source employed, so that the control of the composition of the oxide superconductor which includes at least three metallic elements presents unexpectedly difficult problem.
In addition, these conventionally known methods are not suitable for the thick film formation as well as for the mass production, so that they are disadvantageous for the manufacturing of a large area superconductor.
Moreover, in the thin film obtained by these conventionally known methods, it is possible to obtain the satisfactory crystal orientation property so that the sufficiently high critical temperature Tc and critical current density Jc can be realized, but it is difficult to form a thick film of a desired thickness without sacrificing this crystal orientation property.
On the other hand, there is another conventionally known method of manufacturing an oxide superconductor called the CVD (Chemical Vapor Deposition) method. This CVD method is suitable for the mass production, and has a possibility of realizing a low temperature film formation.
However, in this CVD method, there is a drawback concerning the difficulty to secure the sufficient amount of the necessary alkali metal materials including Ba materials and Sr materials at the appropriate vapor pressure in particular.
There has also been a proposition of a potentially superior method of manufacturing an oxide superconductor called the MOD (Metal-Organic Deposition) method. This MOD method uses a solution in which organometallic complex salts of a plurality of metallic elements to constitute the oxide superconductor are dissolved in an organic solvent, to form a superconductive layer on a substrate by applying this solution on a surface of the substrate and then burning it.
This MOD method has advantages in that it is easy to form a large area superconductor and a superconductive layer of desired thickness and film formation pattern can be manufactured. However, there is a drawback in this MOD method in that it is difficult to obtain an oxide superconductor with a high critical current density.
Now, in any of these conventionally known methods of manufacturing an oxide superconductor, there is an unavoidable problem of realizing a sufficiently high critical current density in a form of a thick film such as a tape member. However, in any of these conventionally known methods, when the film is thickened to a level of a tape member, the critical current density inevitably decreases.
In addition, it has been difficult in any of these conventionally known methods to control the thickness of the film while maintaining a desired superconductor property.
In order to resolve these problems of conventionally known methods, there has been a proposition to raise the critical current density by introducing pinning centers for suppressing movements of magnetic fluxes which are entering into the oxide superconductor at a time of current conduction.
Here, the pinning centers are portions which function to obstruct the movements of the magnetic fluxes due to the Lorentz's force exerted onto the magnetic fluxes entering into the superconductor which would generate the resistances in the superconductor, and it is known that this role of the pinning centers can be fulfilled by deposit particles or grain boundaries formed within the oxide superconductor.
Conventionally, as a method of introducing the pinning centers to the balky oxide superconductor with the composition of Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x, there is a method which incorporates the Y.sub.2 Ba.sub.1 Cu.sub.1 O.sub.x phase into the Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x phase by utilizing the peritectic reaction from a state in which the Y.sub.2 Ba.sub.1 Cu.sub.1 O.sub.x phase and the liquid phase are mixedly present.
However, in this method, in order to introduce the Y.sub.2 Ba.sub.1 Cu.sub.1 O.sub.x phase, there is a need to use a high temperature heat treatment with a temperature over 1000.degree. C. However, when such a high temperature heat treatment is applied with respect to the oxide superconductive layer formed on the substrate, the diffusion reaction at a boundary surface between the substrate and the superconductive layer is promoted, such that the composition of the oxide superconductive layer itself is affected and as a result the lowering of the critical current density is caused.
Also, a conventional method of manufacturing a balky oxide superconductor includes a process for introducing the pinning centers by utilizing the decomposition process from the Y.sub.1 Ba.sub.2 Cu.sub.4 O.sub.8 phase to the mixture of the Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x phase and the CuO phase, but this decomposition process requires the high temperature heat treatment with a temperature over 900.degree. C. under the usual ambient atmosphere with a significant oxygen partial pressure, so that it is difficult to utilize this decomposition process in the method of manufacturing an oxide superconductor on a substrate.